22nd century warp cores were designed as oblong cylinders connected by pylon conduits directly into the warp nacelles. (Star Trek: Enterprise) In the 23rd century, the warp core was not situated in the main engineering. The main warp reaction occurred in a dilithium crystal converter assembly which consisted of two flattened rounded nodules situated directly in front of the warp plasma conduits to the warp engines, which were behind a large metal grate. (Star Trek: The Original Series; ENT: "In a Mirror, Darkly, Part II") By 2270, most Federation warp cores were redesigned to consist of a large warp core unit in the secondary hull with matter and antimatter channeling into the core through vertical conduits, with the resulting energy directed to the nacelles through a horizontal conduit leading out from the rear of the core. (Star Trek: The Motion Picture)